The present invention relates to apparatus adapted for the collection of solar radiation and particularly relates to nontracking, "flat plate" type devices.
A typical flat plate solar collector of the prior art consists generally of a box-like structure having an upper side, insulated back and side members and a single or double pane window or cover sheet (usually glass) adapted to cover the open side of the box. A black absorber of some appropriate material is disposed within the structure for absorbing incident solar radiation which is admitted through the window. Inlets and outlets are provided for passing working fluid in heat exchange relation with the black absorber surface for removing sensible heat therefrom. The heated working fluid may be thereafter utilized in any number of different heating and storage arrangements.
In a typical collector of the prior art, the black absorber may be a sheet of material, usually metal, which is painted black or otherwise treated with appropriate materials to render the surface highly absorbent to incident radiation. The absorber surface may be one of the selective types, which exhibits a high absorptivity .alpha. (low reflectance) characteristic to incident visible radiation and has reasonably low emissivity .epsilon. (high reflectance) for infrared radiation, so that a high percentage of incident solar radiation is absorbed with low re-radiation of thermal infrared to the ambience.
A typical window, (a double pane arrangement of clear glass or plastic with an air space therebetween) provides some insulation from the ambience. By its nature, however, such a window has an upper limit of efficiency, due to residual convection loss caused by circulating air between the panes, conduction through the air and glass, and radiation from the absorber. The efficiency of such flat plate collectors would be enhanced further by evacuation of the air space between the panes. Conventional flat plate collectors with large cover sheets of glass do not lend themselves to evacuation due to the inherent weakness in glass strength caused by tensile stress produced when subjected to a vacuum on one side only.
One way to avoid this strength problem is to use a cylindrically shaped glass envelope to maintain vacuum around an absorber disposed therein. Because glass is strong under compression and due to the compressive nature of such stresses, glass tubing, even with very thin walls, can constitute an ideal structure for an evacuated collector. While plastic might be used, problems of window collapse, outgassing, and water diffusion limit the effectiveness of plastic, most probably to tubular arrangements with low quality vacuum.
In a typical evacuated collector of the type just described, an absorber surface gives up heat to a working fluid passing in heat exchange relation with the absorber. The working fluid may be passed through a U-tube attached to the absorber and connected to a manifold through the tube walls. Such an arrangement, while highly efficient, is costly, since high quality vacuum and glass-to-metal seals are expensive to produce and maintain. Furthermore metal components (usually copper or aluminum) are fabricated from strategic and energy intensive materials.
Another type of collector uses a concentric double wall vacuum bottle as a window. The internal concentric wall of the bottle acts as an absorber, or alternatively an absorber plate is a concentric metal element sleeved within the bottle. Working fluid is passed in heat exchange relation with the absorber in either open or closed circuit relation. Such systems, while effective, suffer from high cost, problems with manifolding, and do not lend themselves to high speed manufacturing technology.
In a closed circuit system, tubes or conduits, in intimate contact with the absorber surface, carry the working fluid (usually liquid). In an open system, working fluid may be trickled over the absorber surface in an open channel or direct contact arrangement. Such structures must be sealed in order to weatherproof and/or maintain vacuum of the system for proper functioning. Furthermore, if a closed system with liquid working fluid is utilized, the fluid must be chosen to reduce the possibility of freezing when the system is not in use (e.g. nights and cold cloudy days), or boiling leading to overpressure when the heat is not being used.
While, the climatic factors of the environment where the collector is to be used, the fuel type to be replaced by the collector and the energy load characteristics (e.g. hot water, heating, cooling) are important considerations when evaluating the design of a solar collector, by far the most important feature to consider is the solar system cost and the solar system performance. If the cost is too high for a given performance level, the solar collector will not become competitive with conventional fuels. Notwithstanding the fact that the future use of conventional fossil fuels is limited, with most recent estimates projecting an exhaustion of known reserves of oil and natural gas within this century, a solar collector having high system performance and relatively low cost must be produced before such systems will become viable alternatives to diminishing conventional fuel supplies. In addition, since nuclear energy sources and coal reserves exist in sufficient quantities to provide the necessary space heating energy requirements for the foreseeable future, a solar collector must compete with these available sources, notwithstanding the fact that the projected cost per BTU of these fuels will probably double or even triple in the near future. For a solar system to be competitive, cost must be sufficiently reduced to provide incentive for its use.
In addition to the foregoing, it is necessary that the amount and cost of materials required for the construction of an efficient solar collector be reduced to a minimum, since relatively large areas of collector surface are necessary to capture the heat sufficient to condition the spaces contemplated. For example, the classical double pane-flat plate collector requires in the order of 3-5 pounds of glass, 2 pounds of copper or other absorber material and about 2-3 pounds of insulation, framing, and encapsulation materials, plus sealants, for each square foot of absorber surface. Consequently if solar collector devices are to become a viable alternative, the material requirements must be substantially reduced, not only because the cost effectiveness will increase, but also because, in the long view, strategic and energy intensive materials such as copper and aluminum should be conserved.
The discussion herein is in terms of cost and performance of the collector based upon square footage of absorber area. In certain cases the total collector structure cost per square foot (insulation, absorber, plumbing, and glazing), is high relative to effective absorber area. In the latter case the structure area is the basis for cost or performance calculations. If, as contemplated in the present invention, the major portion of the collector structure is functionally equivalent to absorber area, the former basis is a valid criteria.
It is important to realize that, every area exerts different constraints on the solar system performance requirements. Total sunlight, average ambient atmosphere (e.g., degree days), percent of heating requirements offset by the solar system, and the length of the heating and cooling seasons, are basic parameters for calculating such variables as total collector surface necessary and the type and volume of storage required, which is compatible therewith. Further, each dwelling or structure requires individualized attention to particular constraints, e.g. the number of windows, exposure, type and quality of insulation, style of dwelling, etc. In this connection, it is important to note that, as the collector structure becomes more complex the manufacturing and materials cost become more difficult to reduce.
The solar collector of the present invention is designed to obviate many of the disadvantages and limitations of the described prior arrangements by providing a simplified structure exhibiting increased efficiency in performance and minimized cost in fabrication, combined with high speed production rates.